DEC DECtape 551 Manual page 75

CIRCUIT SYMBOLS
The block schematics of Digital equipment are multipurpose drawings that combine signal flow,
logical function, circuit type and location, wiring, and other pertinent information. Individual
circuits are shown in block or semiblock form, using special symbols that define the circuit oper-
ation. These symbols are similar to those in the Digital System Modules Catalog but are often
simpl ified. Figure A 1-1 illustrates some of the symbols used in Digital engineering drawings.
LOGIC SIGNAL SYMBOLS
A digital logic signal symbol is shown at the input of almost all circuit symbols to specify the
assertive conditions required to produce a desired output.
All logic signals are either standard Digital logic levels or standard Digital pulses. The standard
Digital logic level is either at ground (0 to - 0.3 v) or at - 3 v (- 2.5 to - 3.5 v). Logic signals
are genera lIy given mnemonic names wh ich indicate the condition represented by assertion of
the signal. An open diamond (--<.» indicates that the signal is a level and that ground rep-
resents assertion; a solid diamond ( . ) indicates that the signal is a level and that- 3 v
represents assertion. All logic levels applied to the conditioning-level inputs of capacitor-
diode gates must be present for either 1 or 3 fJsec (depending on the module used) before an input
triggering pulse is applied to the gate.
The standard Digital negative pulse is indicated by a solid triangle ( - - ...... ) and goes from ground
to - 3 v (- 2.5 to - 3.5-v tolerances). The standard Digital positive pulse, indicated
by an open triangle (---[:», goes either from -3 v to ground or from ground to +2.5 v (+2.3
to +3.0 v). The width of the standard pulses used in this equipment is either 1 .0, 0.4, or
0.07 fJsec, depending on the module and appl ication.
Occasionally, the transition of a level is used at an input where a standard pulse is otherwise
expected and a composite symbol (--.0» is drawn to indicate this fact. The triangle is drawn
open or sol id depending respectively on whether the positive (- 3 v to ground) or the negative
(ground to - 3 v) transition triggers circuit action. The shading of the diamond either is the
same as that of the triangle to indicate triggering on the leading edge of a level, or is opposite
that of the triangle to indicate triggering on the trailing edge. Nonstandard signals are indi-
cated by an arrowhead ( . ) pointing in the direction of signal flow.
Al-2